The invention relates to a synchronous mechanism for a chair, in particular for an office chair, comprising a base unit, a seat support, which is arranged above and moveably on the base unit, a backrest support, which is arranged on the base unit so as to be capable of being pivoted about a pivot axis, which is oriented horizontally and in transverse direction of the chair, and a prestressing device, wherein the prestressing device exerts a prestressing force, which acts upwards on the seat support and forwards on the backrest support, and wherein the seat support is connected to the base unit via a coupling mechanism, which provides for the movability of the seat support relative to the base unit.
The invention furthermore relates to a chair, in particular an office chair.
A chair comprising a synchronous mechanism is known from DE 199 31 099 A1. The movabilities of backrest and seat are mechanically coupled to one another via the synchronous mechanism. The synchronous mechanism comprises a base unit, via which the synchronous mechanism is fastened to a central chair column, wherein the chair column is provided with a star base comprising rollers on the bottom side. The padded seat is arranged on a seat support, which is embodied as seat pad plate and which is connected to the base unit via a coupling mechanism. On the bottom side of the seat pad plate, which faces the bottom, the coupling mechanism comprises bearing supports on which vertically oriented elongated holes are embodied, in which bearing journals, which are arranged on the base unit, are guided. A pressure spring arrangement, which exerts a prestressing force, which acts upwards on the seat pad plate, and a prestressing force, which acts forwards on the backrest, is supported on the bottom side of the seat pad plate. Because of this, it is attained that the prestressing force is a function of the weight of the respective person sitting on the chair, whereby the sitting comfort is to be improved.
The disadvantage of the synchronous mechanism according to DE 199 31 099 A1 is that the prestressing force, which acts on the backrest support, is a function of how the weight, which rests on the seat or the seat support, respectively, is distributed relative to the horizontal. In the event that a user of the chair sits only on the front edge, for example, or substantially on a rear area of the seat, the seat tilts about a horizontal axis, which is oriented in transverse direction of the chair. However, in the event that a user sits on the seat so as to be offset to a lateral area of the seat, the seat tilts about a horizontal axis, which is oriented in longitudinal direction of the chair. It is clear that the seat can be tilted in a random manner about both of these axes by means of such asymmetrical stresses, which equals a floating support of the seat. This floating support of the seat can have the effect, for example, that the seat pad plate is tilted such that at least one bearing journal, which is arranged on the base unit, hits against an upper end of the elongated hole, which is embodied on the respective bearing support, whereby a part of the weight, which rests on the seat, is supported directly by the base unit via this bearing journal, and is no longer available to compress the pressure spring of the pressure spring arrangement and thus to act on the synchronous mechanism for optimally adjusting the prestressing force. A considerable decline of the desired sitting comfort is associated with this.